The Geological Framework of the Accrete Seismic Transect

M L Crawford (Geology Department, Bryn Mawr College, Bryn Mawr, PA 19010; 610-526-5111; e-mail: mcrawfor@brynmawr.edu with other ACCRETE participants from University of Arizona, Beloit,

University of British Columbia, Bryn Mawr, Columbia, Geological

Survey of Canada, Princeton, Purdue, University of Sydney, University of Wyoming, and Virginia Tech

M. L. Crawford and the ACCRETE science team, 1995, The geological framework of the ACCRETE seismic transect. EOS, Transactions, 76, F594.

The ACCRETE transect which crosses the central Coast Mountains orogen follows the maritime border between southeastern Alaska and British Columbia. Relative plate motion for the ACCRETE study area changed from convergent to translational between the mid-Mesozoic and the present. Uplift and erosion along the transect have exposed mid-crustal rocks that record crustal growth by terrane and magmatic accretion. The absence of the upper 15-25 km of the crust and the observed plunge of rocks exposed at the surface to depth along the transect provides a unique opportunity to coordinate seismic and other geophysical studies with directly observed geological features and measured physical properties of deep crustal materials along the transect. A close relationship between deformation and pluton emplacement throughout the orogen recognized by geological studies in the ACCRETE study area has led to conclude that many of the large scale crustal features imaged by our seismic studies result from the fabric imposed on the rocks by high temperature ductile deformation combined with magmatic processes. In addition, this region has experienced igneous activity from the Jurassic to the Recent which provides samples of the deepest crust and upper mantle during orogen evolution. Two periods of voluminous igneous activity resulted in mid-Cretaceous (100-90 Ma) and Paleogene (65-45 Ma) batholiths. The

Cretaceous plutons intruded during the convergent tectonic regime whereas the emplacement of the Paleogene plutons coincides with the inferred change from dominantly orthogonal to transpressive plate motion. The over 800 km long steep to vertical high temperature ductile Coast shear zone coincides with a thermal front between the western side of the high temperature rocks of the Paleogene pluton/ country rock gneiss complex and the previously cooled crust of the mid-Cretaceous orogen. Our seismic data imply a Moho offset of about 5 km (down to the east) across the same boundary. Pervasive mid-crustal ductile deformation and igneous activity have tectonically smeared the sutures between accreted terranes posing the challenge of how to recognize terrane boundaries below upper crustal levels.